Telescopic sighting instruments, such as employed in surveying transits, normally include a spirit or "bubble" level device as a means for establishing a horizontal line of sight in the instrument. Adjusting means are normally provided to enable any necessary correction in the attitude of the bubble vial with respect to the telescope in order to ensure an accurate alignment of the bubble "horizon" with the telescope line of sight.
Typical of such a telescope/level assembly is that shown in U.S. Pat. No. 2,774,275 which includes a support for mounting the bubble vial to the body of the telescope by means of posts located at opposite ends of the vial. Threaded means associated with one such supporting post are usually employed to raise or lower that end of the bubble vial and thus effect the attitude adjustment.
Although simple and straightforward in manufacture and operation, such conventional leveling means nonetheless require highly precise and expensive parts, such as finely threaded adjustment screws. Further, these levels remain particularly susceptible to the effects of weather and accidental mishandling, since, being mounted directly to the telescope body, they are of necessity exposed to such hazards.